Preheat oven to 350 degrees. Spray sides and bottom of loaf pans with non-stick cooking spray. Line bottom of pans with parchment paper and spray the paper, too.
In a medium bowl, whisk both flours, baking powder, baking soda and salt together in a medium bowl.
In a small bowl, combine the sugar and lemon zest. This will help the zest to disperse easily in the batter. Sometimes it will stick to the beaters or clump. The sugar coats the zest.
In a large mixing bowl, blend the sugar and zest mixture, eggs, and lemon juice with an electric mixer until combined.
Add the melted butter to the batter, slowly while mixing.
Then add the sour cream and vanilla and mix until combined.
Add in the flour mixture, a little at a time, until all the flour is combined into the batter. Be sure to not over mix.
Divide the batter evenly between the prepared pans.
Bake in oven for 20 minutes then reduce the oven temperature to 325 degrees and bake another 30-35 minutes, or until toothpick inserted in the center of the loaf comes out clean. Let cool in the pans for 15-20 minutes.